Staff: Mentor

Planck made the assumption that E = nhf while deriving an equation that fit experimental observations on black body radiation. A Google search for "planck black body radiation" finds many web pages that attempt to explain the details, for example

Ok,thank you,but how scientist undestood that?
Also how planck found this constant?

Planck did not find the constant, he made an ansatz (an educated guess). He found that by making this ansatz, that there is some fundamental constant h and that the energy of a vibrational mode (of whatever is vibrating inside a black body) can only be in integer multiples of this constant times the frequency (nhf), he was able to solve the ultra-violet catastrophe. Subsequently, matching with experiment determined the numerical value of this constant.